The present invention relates to ceramic bandpass filters for filtering microwave signals. More specifically, the invention relates to a device for mechanically adjusting the resonance frequency of the ceramic resonators in a ceramic bandpass filter.
Ceramic bandpass filters are used in electrical or electronic devices designed to process signals in the microwave frequency band. Applications for such bandpass filters include Global Positioning System (GPS) receivers, cellular telephones, wireless modems and other remote communication/signal receivers. In conventional ceramic bandpass filters, the center or operating frequency of the filter is determined by the specific application or device incorporating the filter. The ceramic resonators that go into the filters, on the other hand, are constructed with resonant frequencies that are determined by the material composition and the dimensions of the resonators.
In order to match the resonant frequencies of the ceramic resonators with the center frequency of the filters into which the resonators are incorporated, ceramic resonators are routinely modified during the manufacture of the bandpass filters. Ceramic resonators are custom-modified to their specific application. In the prior art, numerous techniques have been devised and used for modifying the resonant frequencies of ceramic resonators.
One such known technique is to grind away a portion of the resonator itself. As the ceramic resonator is ground, its resonant frequency increases. However, grinding as a technique for modifying ceramic resonators is severely limited in that it is only capable of increasing the original resonant frequency of a ceramic resonator; it is incapable of decreasing the original resonant frequency of a ceramic resonator. In addition, grinding is an irreversible process. If a resonator is erroneously ground to a higher resonant frequency, that resonator can never be modified to obtain the lower resonant frequency that was actually desired. The process of grinding ceramic resonators is known as a unidirectional form of tuning a resonator.
Due to the fact that grinding is unidirectional and irreversible, there is a great potential for waste whenever grinding is used in the manufacture of bandpass filters. Bandpass filters in general are simple, conventional devices that, as noted above, have a wide range of applications. Consequently, bandpass filters are produced in extremely large quantities, in excess of several million each year. If a tuning error were to occur in a unidirectional production process, for example by an incorrectly selected center frequency or by a mis-calibrated piece of equipment, large quantities of ceramic resonators would be incorrectly and irreversibly tuned.
Since the ceramic resonators are custom-modified to a specific application, such quantities of incorrectly ground resonators are not necessarily adaptable to other applications. An application calling for the specific resonant frequency at which the resonators were set would have to be readily available in order for the resonators to be used. In addition, resonators by virtue of their construction are not easily recyclable. Considerable time and expense would have to be put into the recycling and reprocessing of the resonators' component materials. As a whole, mistakes in the grinding of ceramic resonators are costly both in raw materials and man-hours of work.
Conventional methods for avoiding the overgrinding of ceramic resonators include using extremely accurate and tightly controlled grinding equipment, and/or grinding the resonators very slowly with close monitoring of the process. Both solutions are costly. Using accurate and tightly controlled grinding equipment requires substantial capital investment in such equipment. Grinding the resonators very slowly with close monitoring slows down production and increases the cost per unit of the resonators. All in all, the methods employed to solve the problems associated with grinding result in additional expense in both equipment and man-hours of work.
Various alternative techniques to grinding have been proposed in the prior art. One such technique is the use of adjusting screws that when positioned with ceramic resonators operate as variable capacitors. This technique is embodied in the devices of U.S. Pat. Nos. 4,268,809 to Makimoto et al.; 4,389,624 to Aihara et at.; 4,628,283 to Reynolds; 4,631,506 to Makimoto et at.; and 5,406,234 to Willems. Though the use of adjusting screws may provide effective means for adjusting the resonant frequencies of the resonators, adjusting screws introduce several notable limitations to the structure and manufacture of resonators using them. First, adjusting screws and the corresponding threaded holes into which the screws are inserted, by the nature of their construction, can only be made so small before they become prohibitively costly and difficult to handle, for example 0.060 in. diameter. As such, using adjusting screws limits the degree to which the overall size of the bandpass filter using the resonator(s) can be minimized. On the other hand, by using a larger adjusting screw, one can increase the range that the resonant frequency can be adjusted. This in turn favors the use of larger screws, but has the drawback of increasing the overall size of the bandpass filter.
Second, adjusting screws are vulnerable to shifting and/or changing position, unless permanently set using a nut, cement or other fixative material. For example, excessive vibration or rough handling may cause the adjusting screws to rotate or fall out of place. Both the placing of a nut with a small adjusting screw, and the applying of a cement or fixative material introduce additional steps into the manufacturing process that increase the cost. Additional materials and components are needed, more man-hours of work are required per unit, and additional equipment must be incorporated to automate those steps. In addition, such steps limit the overall size and design of the bandpass filters. In other words, the bandpass filters would have to be large and accessible enough for the adjusting screws to be inserted, and for either cement or fixative material to be applied to the screws. Due to the limitations discussed above, the use of adjusting screws is not a viable alternative to grinding, especially for applications that demand the miniaturization of components to the greatest degree possible.
Another technique known in the prior art is the use of frequency adjusting devices formed using semiconductor-like processes. For example, U.S. Pat. Nos. 4,987,393 to Yofita et al. and 5,004,992 to Grieco et al. show base plates on which electrode patterns are formed using photolithography. The dimensions of the electrode patterns determine the resonant frequencies of the resonators.
U.S. Pat. Nos. 5,227,747 to Komazaki et al. and 5,304,967 to Hayashi show dielectric blocks or plates that also have electrode patterns formed on them. In order to adjust the resonant frequencies of the resonators, portions of the electrode patterns are etched or trimmed away.
Like the use of adjusting screws, electrode patterns on base or dielectric plates may provide effective means for adjusting the resonant frequencies of the resonators, but nonetheless introduce their own distinct limitations. In particular, if the adjustment of the resonant frequencies is limited to simply attaching a dielectric plate with a prefabricated pattern on it, there is no simple adjustment of the resonant frequencies. Rather, the resonant frequency is determined by the electrical characteristics of the dielectric plate and electrode pattern in combination with the resonators. Using this procedure would require that the process for forming the electrode patterns on the dielectric plates be closely coordinated with the manufacture and/or inspection of the resonators actually used in order to assure that the desired resonant frequencies will result. This procedure increases the per unit cost of the filters by virtue of the added procedures for coordinating the manufacture of the components, and slow down the manufacture of the filters.
If, however, procedures and equipment are added to allow the electrode patterns to be etched or trimmed during manufacture of the filters, the cost of the filters increases due to added time and equipment for etching or trimming the electrode patterns. The etching and trimming of electrode patterns can be either labor intensive, if using technicians equipped and trained to modify electrode patterns, or automated, if using computerized manufacturing equipment. However, either alternative will involve considerable expenditures and highly specialized techniques.
In addition, the electrode pattern process is irreversible. If the resonant frequency set by the etching is not the desired frequency, the electrode pattern cannot be restored. Again, like grinding, the etching or trimming of electrode patterns is a unidirectional form of tuning the resonant frequency of the resonators. As a whole, the use of semiconductor-like processes as discussed above also falls to provide a viable alternative to grinding.